This invention relates generally to apparatus for mounting and driving rotary cutting tools of the type used in drilling, boring and milling and more specifically to a spindle unit used with a motorized driver to cut materials such as metal, plastic, wood, etc., and tool carrying spindle units which are automatically interchanged by an automatic spindle changer. The spindle unit is particularly useful for high speed rotary cutting operations at, for example, 40,000 rpm.
In high speed operations, e.g., at 30,000 to 40,000 rpm, the bearings of spindles typically are expensive and of a large diameter and are provided with special lubrication systems, making them very costly. For a number of applications, the ability to rotate a cutting tool at substantially higher speeds than normal will result in better finish cuts, faster removal of material and less vibration or chatter where a thin web of material is being cut. For example, in cutting aluminum plate to form lightweight, high-strength structures for airplanes or the like, it is desirable to be able to cut out large amounts of aluminum, leaving behind a surface layer reinforced by a network of ribs or webs. To form such pieces in a more economical manner, the speed of cutting and the ability to change tools automatically for the rough and finish cuts is important. High speed cutting is also important when cutting very thin wall parts because, at low speeds, it may be necessary to apply such force by the tool against the thin wall that the thin wall may deflect in order to cut, whereas a very high speed tool may make the same cut without having to be pressed against the thin wall with such a high force as to deflect the thin wall part. Also, generally speaking, the smaller the tool diameter the more desirable it becomes to run the tool faster. The forces applied by the tool to the workpiece may be brought down when the tool speed is brought up. While some individual cutting tools have been operated at very high speeds using special lubricated bearing systems, there is no practical high speed cutting tool system in use today, that uses conventional bearings and lends itself to use with conventional automatic tool changers.
In most common use today are automatic tool changers in which the cutting tool is mounted to a tool adapter or tool holder which is of a large diameter and heavy body and which is stored in a matrix or storage device such as in a magazine, in a rack, or in a line. Typically, the tool holder has a tapered end opposite the tool for seating in a tapered socket in a spindle unit of a machine tool, and has a knob end to be gripped by a gripper or draw bolt mechanism or the like for pulling and clamping the tool holder tightly in the spindle socket. The tool is usually preset in the tool holder and is balanced and checked for length of projection from the tool holder so that once the tool holder is properly seated in the spindle, the tool is ready to cut.
The mass of the typical tool holder and the tool, and their larger size, precludes their use at high speeds for typical machining operations because the inside diameter of the bearing in millimeters (D) is already so large that when it is multiplied by the number of revolutions per minute (N) of the bearing, a DN value of 1,000,000 results, which is the usual upper limit for conventional bearings used in machine tools. More specifically, when the multiplication product of DN is greater than 1,000,000, special precautions must be taken regarding bearing materials, lubrication and mounting to minimize the effects of friction and heat, resulting in expensive and impractical conditions for use of the bearing for a machine tool application. The present invention is directed to using readily available bearings in a spindle to support a tool for high speed rotation where the product of DN is approximately 1,000,000. When reference is made herein to "readily available" bearings, it is intended to include ball bearings commercially available for high speed applications, rather than low speed ball bearings or high speed bearings having special cooling facilities. By keeping the bearing diameters small, the rotational peripheral velocity of the ball bearings may be kept low while the rotational velocity of the tool may be increased to a high velocity. To this end, the present invention eliminates the large size and mass of the conventional tool holder and yet allows for automatic tool changing with conventional types of tool changers.
The tool holder and the draw bolt mechanism which retains the tool holder are of considerable mass and this mass must be rotated by the machine spindle along with the tool itself. This large mass, and the fact that it is cantilevered from the forward end of the machine spindle, is also a deterrent to high speed operation of the tool. The proper alignment of the tool and the tool holder so that this mass rotates coaxially with the spindle shaft becomes more important at elevated speeds. Any eccentricity or misalignment has serious consequences at such speeds. This problem is magnified by the fact that this eccentric mass is centered at a point spaced well forward of the front spindle bearing. Also, balancing of the tool and tool holder is more of a problem at high speed than at low speed. With the vibrations produced by any eccentricity of the tool and the tool holder applying a cantilever loading on the forward end of the spindle shaft, there is a further amplification of these vibrations. Additionally, improper mating of the tool holder to the spindle may result from obstructions between the mating faces such as dirt, metal chips or coolant residue. This in turn would cause further unbalance and vibration.
The tool holders have typically included a sharply tapered shank which cooperates with the tapered socket at the end of the spindle shaft to try to obtain concentricity of the tool and tool holder with respect to the spindle axis. This tool holder structure either increases the size of the front spindle bearing or causes the bearing to be located further inward from the end of the spindle shaft. In the latter case, there results a greater overhang or cantilevering of the spindle shaft with the accompanying eccentricity and increased bearing load problems. For these and various other reasons, the present approach to mounting, drilling, boring and milling tools in large tool holders, which are mounted for rotary movement extending from the outer end of a spindle shaft, is not satisfactory when applied to machining applications requiring high rotational speeds.
Another undesirable attribute of the presently used high mass tool holder relates to increased machine time required to accelerate the tool holder to its desired cutting speed and decelerate it upon completion of a cutting operation. In automated machining operations, many tools are used in successive machining operations so the time lost in accelerating the high mass tool holder up to and down from the desired cutting speed may be a very substantial portion of the overall machining time. Any reduction in the mass of the tool holder would result in shorter acceleration times and an improvement in machine efficiency.
The assignee of this application has heretofore developed a master head for very large milling machines to which a variety of individual, very large spindle units are assembled. Typically, the spindle units weigh several thousand pounds and are driven at slow or conventional speeds and cannot be operated at those high speeds desired for the spindle units of the present invention. This master head approach represents a departure from the concept of automatically changing tools to that of automatically changing spindle units. The master head and interchangeable spindle units are disclosed in the U.S. Pat. to Lewis et al. U.S. Pat. No. 4,709,465, which is assigned to the same assignee as the present application. The spindle units disclosed in the Lewis et al. patent are not light in weight and are not suited to the high rotational speed operation, as discussed above.
It is known in the art to use lightweight chucks and collets for mounting various types of rotary cutting tools directly on a shaft or spindle of a machine. One such collet type mounting spindle is disclosed in the U.S. Patent to Wall U.S. Pat. No. 2,286,292. Such devices are not suited for high speed industrial applications of the type described above for several reasons. If the tool is to be used in an automated machine in which the tool is selectively applied to a motorized driver to perform a particular cutting operation, some type of the tool holder is used. The tool is selectively mounted in the tool holder so that a desired depth or length of cut is achieved when the tool is automatically placed in its driving position in each cycle. Accordingly, some type of tool holder is required to permit the automatic loading and positioning of the tool. In addition, tool holders are needed to permit such automated machine to accommodate a wide variety of tool sizes which are driven by the same motorized driver.